A large body of evidence obtained from studies of the distribution and pharmacodynamics of 99mTc-HEDP shows that this compound is selectively adsorbed at sites of high metabolic activity in bone, and particularly at sites of bone disease where amorphous calcium phosphate is abundant. This selectivity may be used to irradiate these disease sites if a radionuclide with the proper physical characteristics could be incorporated into the HEDP molecule in high specific activity without altering the distribution of the product. A suitable radionuclide is phosphorus-33, a pure-beta emitter, having a half-life of 25 days and an end point energy of 0.25 MeV. The limited range of these particles assures the deposition of radiation dose principally to cells requiring therapy on bone surfaces. The plan of the study is to synthesize HEDP labeled with radiophosphorus (32P for animal studies, 33P for trials in model systems) and observe the distribution, retention and excretion of 33P-HEDP in laboratory animals for the purpose of estimating radiation dose to normal tissue. Long-term concentration of labeled HEDP will be observed in bone cancer model systems and in cancerous bone tissue of human subjects. Trials in model cancer systems of 33P-HEDP will permit assessment of a benefit/risk ratio and a comparison with alternate modes of treatment and will provide the basis for decision on the applicability of human cancer therapy trials.